Its not the "general opinion" - its fact, cycling will improve your cycling, rowing will improve your rowing, theres no two ways about it, you can make small "cross-over" improvements by the fact that if you are unfit in the first place than your cardiovascular system will be more efficient after a period of training from either, but if you're anything other than a beginner it'll be worthless.
The only help I can see is for a cycling machine to maintain body composition/improve it, and maybe help with leg strength if the right training is done.